The invention relates to an optical system for concentrating light from diode laser arrays.
Diode-pumped solid state lasers are of tremendous interest because of their excellent beam quality, narrow linewidth, high reliability, and potential for high efficiency. Typically, a linear diode laser array (pump array), consisting of a row of many small diode lasers, is used to pump these devices in one of two ways, either transversely or longitudinally. When used for transverse pumping, the pump array is aligned parallel to the axis of the solid state laser mode so that each of the individual beams of the many small diode lasers enters the solid state laser transversely and pumps a different lengthwise portion of the solid state laser. This technique, although allowing the addition of many pump arrays around the solid state laser, is intrinsically limited to low efficiency since pump energy is necessarily absorbed by portions of the solid state laser crystal which do not subsequently contribute energy to the lasing mode.
Longitudinal pumping involves combining all of the individual beams of the pump array in such a way as to direct all beams down the axis of the solid state laser collinearly with the lasing mode. In an idealized sense, longitudinal pumping has the advantage of being much more efficient than transverse pumping since pump energy is deposited only into the active or lasing volume of the solid state laser crystal. However, when conventional optics are used to achieve longitudinal pumping with an incoherent spatially distributed source, the size and divergence of the solid state laser mode limits the extent of the pump array which can be efficiently channelled in the active volume. Using longer pump arrays, for instance, would result in portions of the pump array having incident angles upon the laser crystal which are too large and thus not well matched to the laser mode.
To compensate for the lower efficiencies of transverse pumping, considerable effort has been spent on making large one-dimensional laser diode arrays. As a consequence of that effort, large, high power pump arrays (up to a centimeter in size) are now commercially available for transverse pumping applications. It is desirable to find a practical way to also employ these commercially available large pump arrays for longitudinal pumping and thus achieve even higher levels of performance. In one approach to accomplishing this, fiber optics were used in an experimental set-up to combine the outputs of several linear arrays into a longitudinal pump. The fiber optic system, however, was difficult to fabricate and use. In addition, fiber coupling losses reduce the efficiency of the system.